The present invention is directed to a method for controlling a web tension in a rotary printing press. The web passes through at least one printing unit and the web""s tension is adjusted before that at least one printing unit.
EP 09 51 993 A1 discloses a register-preserving actuation for a rotary printing press, in which a longitudinal stretch of the printing web is ascertained from web tension values and operational data of the drive units and is compensated for by adjusting the circumferential register on the cylinders or the register rolls. A change in the lateral stretch, which is ascertained by a sensor for detecting web width, is restored to its original value by changing the reference value of the draw roller, which is adjusted to a constant web tension.
U.S. Pat. No. 3,025,791 A discloses a method for controlling the drive units of a printing press with the aim of achieving a constant web stretch. The measurement of the web stretch in this instance takes place in close proximity to the first printing unit, through comparison of the angular position of the printing unit and then the position of a mark on the print stock. A change in the relative position between the angular position of the printing unit and the position of the mark results in a tension change for the print stock web in the infeed unit.
DE 198 34 725 A1 discloses a number of methods for controlling web tension. The web tension is measured, and with this measurement the tension is restored to a reference value or a reference range for the tension or is restored to a mathematically calculated speed for a lagging or retarding or an advancing of one drive unit in relation to a second drive unit. The web tension in this instance is kept constant by use of a simple web tension control, a web tension control that has been expanded to include a presettable reference value for the speed, or a lag controlxe2x80x94with or without DROOP behavior.
DE 29 51 246 C2 discloses a device used in a printing press, which makes it possible to adjust the speeds of a paper web at different points in relation to one another, in an example, the drive unit of the printing unit on the one hand and that of the draw roller on the other. In this case, by use of motor shaft encoders, impulses that correspond to the speed of the web can be detected and are supplied to a motor control circuit
The object of the present invention is to provide a method for controlling a web tension in a rotary printing press.
The object is attained according to the invention by the measurements of web speed at first and second locations along the path of web travel. A change in the difference in these speeds is the result of a change in web tension. This change in web tension can then be controlled. The change in web tension can be made before the web enters a first printing unit in the rotary printing press.
The advantages that can be achieved with the present invention are comprised particularly in the fact that by measuring and comparing the circumferential speeds of the last printing unit and of the subsequent draw roller, a number of objects can be inexpensively fulfilled, without incurring substantial additional expenses related to measurement techniques. The stretch or tension of the web is kept constant despite fluctuating web properties and operating parameters.
In particular, when starting up a printing unit, a printing tower, or an entire multi-web rotary printing press, there is often a printing startup without the use of water and/or ink, i.e. with a dry web. Before a printing startup of the cylinders occurs, a largely uniform web tension prevails before and after the printing units since the web is not placed under tension by the printing units. For the web travel, though, it is advantageous if after printing startup, but during the operation of the press without water and/or ink, the web tension is also approximately the same before the first printing unit and after the last printing unit. This is the case if the circumference speeds of the last printing unit and the subsequent draw roller are approximately the same, which can be advantageously assured by the present invention.
When the water and ink are switched on, the tension or stretch behavior of the web changes as it passes through the printing zones. This results in a tension reduction after the last printing unit since the subsequent draw roller is, for the time being, still traveling with the same circumferential speed as the last printing unit. So that a trouble-free entry of the webs into the hopper of the web former and folder is assured in multi-web operation, in order to achieve the necessary graduation in the web tension, after the water and/or ink are switched on, but before the print run speed has been reached, the appropriate web tension level of the webs are matched to each other, if possible only by adjustments to the infeed unit.
The paper web stretches under the influence of the wetting agent and/or the ink in both the longitudinal and lateral directions of the web with respect to the web transport direction. Particularly in multicolor printing, where there are free path lengths between adjacent printing zones, this paper web stretching is reflected in a fan-out effect or a spreading of the print image or of several adjacent print images as they continue on their way through the printing zones. Provided that this so-called fan-out effect remains virtually constant over time in each printing zone, the effect can be at least partially compensated or counteracted by, for example, printing formes which are disposed so that they diverge in a corresponding manner. The stretch behavior of the paper web, however, is subject to numerous influences, such as the tension or stretch characteristics of the respective paper and consequently the prevailing tension, the current moisture, the moisture sensitivity, the penetration behavior, and even the position of the roll as it is produced in the reel, which is reflected, for example, in a variable winding hardness.
The stretch, both the longitudinal and lateral stretch of the paper web, is therefore not steady or constant due to inconstant paper properties of the unrolling paper web itself and due to changing and partially fluctuating operating parameters in the printing press. For example, the paper web on the roll may have been subjected to an uneven winding in its manufacture, may exhibit a location-dependent fluctuation in its modulus of elasticity, or other irregularities. As a rule, these properties vary greatly from paper type to paper type. Even with the same paper type, the properties can vary considerably from roll to roll. On the other hand, a fluctuating web tension, changing printing speeds, fluctuations in the wetting of the web, or a roll change influence the stretch of the paper web so that the longitudinal stretch and the lateral stretch of the web in relation to the transport direction are not steady or constant over time.
When there are fluctuations in the properties of the web and also changing operating parameters, for example in the wetting of the web, the mathematical correlation of a tension or a stretch characteristic curve is abandoned and that curve changes to a temporarily unknown characteristic curve. Furthermore, if only a constant tension is set, this results in a deviation in the stretch and in the effective unwinding length of the print image on the web B, which results in errors in the image length on the web B and also in the circumference and/or side register. The method for controlling web tension in accordance with the present invention eliminates this disadvantage. The web stretch is kept constant even with the above-mentioned inconstant web properties and/or operating conditions.
The method in accordance with the present invention advantageously assures a more reliable starting procedure, establishes a normal setting, and compensates for additional fluctuations in the stretch in the longitudinal and/or lateral directions of the web.
In a preferred embodiment, during the starting phase, which typically occurs without ink and water, the draw roller disposed after the last printing unit is driven in a speed-controlled fashion in relation to a printing unit. During print running, this draw roller is torque-controlled in respect to a presettable reference value. The speeds of the draw roller and of the printing unit, that continue to be measured during a print run, are used to control the infeed unit. After print run speed has been reached, the difference between the circumference speed of the draw roller, which as a rule increases during print running, and the circumference speed of the printing unit constitutes a reference value. A change in this difference or in this reference value indicates a change in the web tension or stretch, not only in a longitudinal stretch but also in a lateral stretch of the print stock web, and consequently a change in the image length on the web B and/or in the lateral and longitudinal register. The torque control produces a temporary speed change of the drive unit of the draw roller. The relative speed change in turn then controls the infeed unit, which finally permits the torque-controlled draw roller to return to its xe2x80x9cnormal operationxe2x80x9d. A change in the stretch or in the tension of the paper web is measured after the last printing unit. However, a controlling action is executed at the infeed unit, which action defines the overall tension level of the paper web. Advantageously, there is no direct feedback to the draw roller situated after the printing unit. Instead, there is a change in the overall tension level at the infeed unit. A particularly advantageous feature of the present invention is the fact that the web tension after the draw roller is kept largely constant, which can be achieved through combination of the torque-controlled draw roller with the control of the infeed unit as to the difference between the circumference speeds, without additional expenses related to measurement and control techniques.
It is advantageous if the tension or the stretch change in the web is ascertained at the end of the printing tower or after the last printing unit. This offers a good insight for the additional processing steps as to the overall change and makes it possible to execute a countermeasure to achieve a constant tension of the web for the subsequent paths of the web. In this sense, it is also advantageous that the control of the web tension does not take place in the vicinity of the measurement zone, but at the beginning of the web. This establishes a base level for the course of the tension, without causing significant changes in the web tension in the superstructure, in particular before the hopper infeed roller of the web former and folder.
A virtually constant portion in the longitudinal stretch, for example for the purpose of preadjusting the printing press for the expected or measured operating conditions and paper conditions, can be compensated, for example by register adjustment in the cylinders, by the use of register rolls, or by other devices.
It is also advantageous that both the first requirement discussed previously for a speed-controlled startup, and, after the switching on of the water and ink, the second requirement for a control of changes in the fan-out effect or the lateral register and in the image length when the web tension after the draw roller is kept largely constant, are fulfilled without requiring, for example, additional systems for image detection or the like in order to preserve the lateral register.